[1] Geological repositories subject to the injection of large amounts of anthropogenic carbon dioxide will undergo chemical and mechanical instabilities for which there are currently little experimental data. This study reports on experiments where low and high P co 2 (8 MPa) aqueous fluids were injected into natural rock samples. The experiments were performed in flow-through triaxial cells, where the vertical and confining stresses, temperature, and pressure and composition of the fluid were separately controlled and monitored. The axial vertical strains of two limestones and one sandstone were continuously measured during separate experiments for several months, with a strain rate resolution of 10 À11 s À1. Fluids exiting the triaxial cells were continuously collected and their compositions analyzed. The high P co 2 fluids induced an increase in strain rates of the limestones by up to a factor of 5, compared to the low P co 2 fluids. Injection of high P co 2 fluids into the sandstone resulted in deformation rates one order of magnitude smaller than the limestones. The creep accelerating effect of high P co 2 fluids with respect to the limestones was mainly due to the acidification of the injected fluids, resulting in a significant increase in solubility and reaction kinetics of calcite. Compared to the limestones, the much weaker response of the sandstone was due to the much lower solubility and reactivity of quartz in high P co 2 fluids. In general, all samples showed a positive correlation between fluid flow rate and strain rate. X-ray tomography results revealed significant increases in porosity at the inlet portion of each core; the porosity increases were dependent on the original lithological structure and composition. The overall deformation of the samples is interpreted in terms of simultaneous dissolution reactions in pore spaces and intergranular pressure solution creep.

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Résumé -Propriétés pétrophysiques du Dogger carbonaté dans le Secteur PICOREF (Sud Oil & Gas Science and Technology -Rev. IFP, Vol. 65 (2010), No. 3, pp. 405-434 Copyright © 2010, Institut français du pétrole DOI: 10.2516/ogst/2010002 (1) PIégeage du CO 2 dans des REservoirs géologiques en France. CO 2 Storage in the Struggle against Climate ChangeLe stockage du CO 2 au service de la lutte contre le changement climatique

Abstract. When a reactive fluid circulates inside a porous medium it can dissolve some minerals if equilibrium is not reached and modiy the porosity and permeability. The positive feedback between fluid transport and mineral dissolution lead to complex reaction front morphologies such as fingers. Our study is carried out with two objectives: l) to evaluate experimentally these processes at a decimeter scale, 2) to compare the experiment to a numerical model of water-rock interaction. The experiment consists of a two-dimensional porous medium that allows for the dissolution of halite under an imposed fluid flow. The numerical code used solves the equations of reaction and transport in a porous medium. Both experiment and numerical simulation indicate the development of an instability whose propagation rate depends on the rate of water injection raised to a 2/3 power.